Chemical reactions are processes where substances (reactants) transform into new substances (products) with different properties. In RRB Group D exams, this topic appears in 2-3 questions every year, making it extremely important. Core Concept: A chemical reaction involves breaking old bonds and forming new bonds. The atoms remain the same, but their arrangement changes. For example, when iron rusts, iron atoms combine with oxygen atoms to form iron oxide.
Mass is conserved - total mass of reactants equals total mass of products (Law of Conservation of Mass)
Atoms are neither created nor destroyed, only rearranged
Energy changes always occur - reactions either release heat (exothermic) or absorb heat (endothermic)
Chemical equations must be balanced
- Balancing: Count atoms on both sides, make them equal
RRB Group D typically asks about balancing equations (40%), types of reactions (30%), and real-life examples (30%). Questions often involve identifying reaction types or completing word equations. Shortcut for Balancing Equations: Start with the most complex molecule first. Balance metals, then non-metals, then hydrogen and oxygen last. Use the 'LCM method' - find LCM of atoms appearing on both sides.
Count Fe atoms - Left: 1, Right: 2 (unbalanced)
Put coefficient 2 before Fe: 2Fe + O2 → Fe2O3
Count O atoms - Left: 2, Right: 3 (unbalanced)
Put coefficient 3 before O2 and 2 before Fe2O3: Fe + 3O2 → 2Fe2O3
Balance Fe: 4Fe + 3O2 → 2Fe2O3 Answer: 4Fe + 3O2 → 2Fe2O3 Worked Example 2: Identify reaction type - CaCO3 → CaO + CO2
Count reactants - One compound (CaCO3)
Count products - Two substances (CaO and CO2)
One reactant breaks into multiple products = Decomposition reaction
Heat is required, so it's thermal decomposition Answer: Thermal decomposition reaction
for Reaction Types: Remember 'CORD' - Combination (A+B→AB), Oxidation-reduction (electron transfer), Replacement (single/double), Decomposition (AB→A+B). Most RRB questions test these four types.
#1: Students forget to balance equations and write wrong coefficients. Always verify by counting atoms on both sides after balancing. Another frequent error is confusing physical changes (like melting) with chemical changes (like burning).
Chemical changes form new substances; physical changes don't. Another exam trick: When identifying precipitation reactions, remember that most carbonates, sulfides, and hydroxides are insoluble except those of Group 1 metals and ammonium. This helps predict which combinations will form precipitates. For energy changes, remember: Combustion and respiration are always exothermic. Photosynthesis and electrolysis are always endothermic.
This pattern appears frequently in RRB papers.
12graded MCQs · easy to hard · full solution & trap analysis